Osteoarthritis (OA) is the most common form of arthritis, a leading cause of disability in older Americans, and a growing economic burden to our society. Characterized by degradation of articular cartilage, synovitis, subchondral bone thickening, osteophytes, and other joint changes, OA is extremely painful and debilitating. Due to the aging population in the USA, there is an urgent need to provide new solutions that prevent osteoarthritis and/or promote cartilage healing. The experiments outlined in this proposal are designed to validate Phlpp1 as therapeutic target and could be rapidly translated into a therapeutic approach for OA. PHLPP1 (pleckstrin homology domain leucine-rich repeat protein phosphatase, flip) is an intracellular phosphatase that terminates numerous signaling pathways, including Akt and PKC, to repress proteoglycan and type II collagen synthesis, and affect proliferation, differentiation, and survival. We discovered that PHLPP1 is highly expressed in articular cartilage from osteoarthritis (OA) patients. We hypothesize that PHLPP1 promotes chondrocyte hypertrophy during OA progression by virtue of its ability to regulate Akt and other signaling pathways. The objectives of this project are to define how PHLPP1 contributes to OA progression and skeletal development using both human tissues and animal models and to validate Phlpp1 as a therapeutic target for OA. The specific aims of this project are to: 1) Define the role of Phlpp1 in OA progression by performing DMM surgery on skeletally mature Phlpp1-/- mice and monitoring OA progression through functional testing, imaging and histology. Phlpp inhibitors will also be tested in the DMM model. 2) Determine the role of Phlpp1 in endochondral bone and joint formation by examining bone and cartilage development in Phlpp1-/- animals with microCT imaging, histomorphometry, and in situ hybridization; in vitro differentiation assays with primary chondrocytes, osteoblasts, and osteoclasts from Phlpp1-/- and wildtype mice will also be performed in the presence or absence of Phlpp inhibitors; and 3) Define the epigenetic events and soluble factors controlling PHLPP1 expression in OA cartilage by testing the hypothesis that PHLPP1 expression is epigenetically controlled by DNA demethylation in OA cartilage and/or by inflammation-induced release of Hdac co-repressors. The significance of this work is that PHLPP1 is a new and druggable target whose activities and/or expression could be controlled to reset chondrocyte signaling pathways and slow OA disease progression. The work is innovative because the role of PHLPP1 in cartilage development and disease has never been explored. Our team possesses necessary reagents and expertise and thus is uniquely positioned to efficiently complete this project. Our results will have an impact because they will validate PHLPP1 as therapeutic target and could be rapidly translated into a clinical option for millions of Americans suffering from OA.